# CVE-2026-40109: Flux notification-controller GCR Receiver missing email validation allows unauthorized reconciliation triggering

> Attackers with webhook URL access can bypass email validation to trigger unauthorized Flux reconciliations, though impact is limited by idempotency.

- **Severity:** Low 3.1 (`CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 9 Apr 2026
- **Updated:** 14 Apr 2026
- **Also known as:** GHSA-h9cx-xjg6-5v2w, GO-2026-5419

## What is CVE-2026-40109?

CVE-2026-40109 is a low-severity authentication bypass vulnerability in fluxcd notification-controller, published on 9 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

Flux notification-controller is the event forwarder and notification dispatcher for the GitOps Toolkit controllers. Prior to 1.8.3, the gcr Receiver type in Flux notification-controller does not validate the email claim of Google OIDC tokens used for Pub/Sub push authentication. This allows any valid Google-issued token, to authenticate against the Receiver webhook endpoint, triggering unauthorized Flux reconciliations. Exploitation requires the attacker to know the Receiver's webhook URL. The webhook path is generated as /hook/sha256sum(token+name+namespace), where the token is a random string stored in a Kubernetes Secret. There is no API or endpoint that enumerates webhook URLs. An attacker cannot discover the path without either having access to the cluster and permissions to read the Receiver's .status.webhookPath in the target namespace, or obtaining the URL through other means (e.g. leaked secrets or access to Pub/Sub config). Upon successful authentication, the controller triggers a reconciliation for all resources listed in the Receiver's .spec.resources. However, the practical impact is limited: Flux reconciliation is idempotent, so if the desired state in the configured sources (Git, OCI, Helm) has not changed, the reconciliation results in a no-op with no effect on cluster state. Additionally, Flux controllers deduplicate reconciliation requests, sending many requests in a short period results in only a single reconciliation being processed. This vulnerability is fixed in 1.8.3.

Weakness: CWE-345 (Insufficient Verification of Data Authenticity), CWE-287 (Improper Authentication)

## Which versions are affected by CVE-2026-40109?

3 open source packages ship an affected version, including github.com/fluxcd/notification-controller.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/fluxcd/notification-controller` | Go | All versions | 1.8.3 |
| `github.com/fluxcd/notification-controller` | Go | All versions | 1.8.3 |
| `github.com/fluxcd/notification-controller` | Go | All versions | 1.8.3 |

## How do I fix CVE-2026-40109?

Upgrade github.com/fluxcd/notification-controller to 1.8.3 or later.

## Is CVE-2026-40109 being exploited?

There is no known exploitation of CVE-2026-40109 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-40109 and fix it if it is affected.

What it is: CVE-2026-40109 is a low-severity authentication bypass vulnerability in fluxcd notification-controller, published on 9 Apr 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-40109 in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/fluxcd/notification-controller (Go): affected all versions; fixed in 1.8.3
- github.com/fluxcd/notification-controller (Go): affected all versions; fixed in 1.8.3
- github.com/fluxcd/notification-controller (Go): affected all versions; fixed in 1.8.3

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-40109/ (Markdown: https://insights.sbomify.com/CVE-2026-40109/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-40109
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40109
- OSV: https://osv.dev/vulnerability/CVE-2026-40109

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-40109/
Summaries are generated and may contain mistakes; the official description above is the record.